The present invention relates to a main shaft gear mechanism for automobile (passenger car, truck, bus or the like) speed changers, and a tapered roller bearing used therefor.
For example, an automobile speed changer has a role to change the speed change ratio to meet widely varying travel conditions, thereby changing the engine torque so as to make smooth comfortable travel possible. Generally, the speed changer has to satisfy such requirements as proper speed change ratio, sufficient strength, durability, reliability, high power transmission efficiency, silence, and small-size and lightweight.
An example of this speed changer is a synchromesh type speed changer shown in FIG. 12. In this speed changer, a main shaft 11 and an auxiliary shaft 12 which are disposed parallel with each other with a predetermined spacing therebetween are supported for rotation in a mission case (not shown), said main shaft 11 being operatively connected to an output shaft (associated with the driving wheels), the auxiliary shaft 12 being operatively connected to an input shaft (associated with the engine).
The auxiliary shaft 12 is integrally provided with an auxiliary shaft gear 13, while the main shaft 11 has a main shaft gear 16 (which serves also as a bearing outer ring) rotatably mounted thereon through a bearing inner ring 14 and needle rollers 15 and constantly meshing with the auxiliary shaft gear 13. One side of the main shaft gear 16 is formed with spline teeth 17 and a cone 18, and a hub 19 is disposed close to the end surface of the cone 18 and integrally engagingly connected to the main shaft 11. A synchromechanism 20 is interposed between the hub 19 and the cone 18, and the outer periphery of the hub 19 has a sleeve 21 axially movably spline-connected thereto.
In the state shown in this figure, receiving the rotation of the auxiliary shaft gear 13, the main shaft gear 16 is idling with respect to the main shaft 11. On the other hand, when the sleeve 21 is moved axially rightward from the state shown in the figure, it meshes with the spline teeth 17 of the main shaft gear 16 through the synchromechanism 20, whereby the connection between the main shaft gear 16 and the main shaft 11 is established. Thereby, the rotation of the auxiliary shaft gear 13 is reduced by the main shaft gear 16 with a predetermined speed ratio and transmitted to the main shaft 11. During this speed change, the main shaft gear 16 rotates in synchronism with the main shaft 11 and the bearing inner ring 14.
In this connection, during speed change, the synchronous rotation of the main shaft gear 16 and the bearing inner ring 14 causes the needle rollers 15 to assume a stopped position on the raceway surfaces of the two members 14 and 16. On the other hand, repetitive action of external vibrations or the like could cause repetitive slight slippage between the needle rollers 15 and the raceway surfaces, so that the phenomenon called fretting in which the contact surface wears due to the relative repetitive slight slippage may become a problem.
In an arrangement intended to prevent fretting, the Parker process (phosphating) is applied to the raceway surfaces of the main shaft gear 16 and the bearing inner ring 14, and the rolling contact surfaces of the needle rollers 15 to reduce the wear resistance between the needle rollers 15 and the raceway surfaces. However, wear of Parker process coatings may make it impossible to expect a long-term satisfactory fretting-preventive effect.
I have previously proposed, as means capable of long-term prevention of fretting on the raceway surfaces of the main shaft gear and the bearing inner ring and on the rolling contact surface of rolling element, an imbalance means in the form of the irregular circumferential disposition of rolling element, the circumferential weight imbalance of a cage holding rolling element, or rolling element weight inequality (Japanese Kokai Patent Publication Heisei 9-292008). Depending on the size of variation in rotation (variation in speed) or vibration, however, it might be difficult to attain a satisfactory fretting-preventive effect.